Metallic and semiconductor elements such as iron, aluminum, copper, silicon, etc. are very unlikely to exist as simple substances in the natural world, and those elements mostly exist as oxides or other compounds. Thus, in order to use the metallic or semiconductor elements as structural materials, conductive materials, semiconductor materials or the like, it is often necessary to reduce their oxides or the like to simple substances of the metallic or semiconductor elements.
Further, desired metallic or semiconductor materials as just reduced from their oxides or the like often contain impurities at concentrations that are improper for such uses of the materials as mentioned above. As such, adjustment of the impurity concentrations, which is reduction in many cases, is generally carried out. Such a process of reducing the impurity concentrations is called purification.
In other words, the purification means extraction of impurities in their different state from a simple substance of a metal or a semiconductor, which is carried out by appropriate physico-chemical processing that is suitable for physico-chemical properties of the metallic or semiconductor matrix and/or the impurity elements.
In the case of copper that is a typical material for electric wire, for example, so-called unidirectional solidification is used to increase purity of the copper to obtain a wire material having a low electric resistance value. In the unidirectional solidification process which utilizes the fact that the ratio of an impurity concentration in a solid state to that in a molten sate of a metallic or semiconductor material, i.e., a so-called segregation coefficient of the impurity, is generally small in the equilibrium state, the solidification is conducted at a slow rate near the equilibrium state so as to reduce the impurity concentration in the solidified copper.
In the case of silicon that is most widely used as a semiconductor material, metallurgical grade silicon having a purity of 98% or more, which is obtained by reducing silica, is converted into gas such as silane (SiH4) or trichlorosilane (SiHCl3) which is then reduced with hydrogen in a bell jar furnace to obtain polycrystalline silicon having a purity of 11N. From the polycrystalline silicon, single-crystalline silicon is grown to obtain silicon wafers to be used for electronic devices such as LSIs. In order to satisfy the requirements for use in electronic device applications, very complicated production processes and strict management thereof are required, which inevitably increases the production costs.
Meanwhile, in recent years, there is a rapidly increasing demand for silicon as a material of solar cells, because of increased concern about energy and environmental problems such as depletion of fossil fuels, global warming and others. In the case of silicon for use in solar cells, the purity necessary for desired performance of the solar cell is in the order of 6N. This means that silicon as being out of the standard for use in electronic devices, which has conventionally been used as the material for solar cells, has excessive quality as the material of solar cells from the standpoint of purity.
To date, the produced amount of silicon as being out of the standard for use in electronic devices has been greater than the demanded amount of silicon for use in solar cells. It is expected that, in the near future, the demanded amount of silicon for use in solar cells will exceed the produced amount of silicon as being out of the standard for use in electronic devices, and thus there is a strong demand for establishment of a technique enabling inexpensive production of silicon as the material for solar cells. Recently, attention is drawn to a method of purifying metallurgical grade silicon having a purity of about 98% as mentioned above by a metallurgical method using solidification segregation or the like.
Iron, aluminum, titanium and the like are contained in relatively large amounts among the impurity elements of heavy metals or semiconductors in metallurgical grade silicon. The impurity concentrations in metallurgical grade silicon are typically as follows: 100-5000 ppmw for iron, 100-2000 ppmw for aluminum, and 1-10 ppmw for titanium.
It is known that the above-described segregation coefficients of the heavy metal elements in silicon are small. For example, according to the 1997 year report of the SOGA (Solar-Grade Silicon Technology Research Association), the segregation coefficients of iron, aluminum and titanium are 6.4×10−6, 2.8×10−3, and 7.37×10−6, respectively. The concentration of each of these impurity elements can be reduced to a level of 0.1 ppmw or less required in the material for solar cells, by conducting purification utilizing solidification segregation two or three times, as represented by the unidirectional solidification described above.
The solidification segregation method is advantageous in that a number of impurity elements can be processed at the same time. In the unidirectional solidification, however, a molten metal or semiconductor introduced into a mold is solidified at a slow rate near the equilibrium state as described above, and thus the processing rate is very slow.
Further, in the resultant ingot, a portion obtained in the earlier stage of solidification has impurity concentrations smaller than those before the solidification processing (which portion is hereinafter referred to as the “purified portion”), while a portion obtained in the later stage of solidification has impurity concentrations higher than those before the solidification processing (which portion is hereinafter referred to as the “impurity-concentrated portion”). The ratio of such impurity-concentrated portion with respect to the entire solidified ingot is about 20% to 50%, though it varies depending on the impurity concentrations before the solidification processing, the speed of solidification, the degree of stirring of the molten metal or semiconductor, and the like. That is, in order to carry out the solidification processing two or three times, it is necessary to eliminate the impurity-concentrated portions of considerable amounts by cutting them off.
Further, an additional crushing step is necessary in order to introduce the purified portion again into the furnace for melting. Such cutting and crushing steps can be performed on the solidified ingot only after it is cooled near a room temperature, which takes several to several tens of hours. As such, conducting the unidirectional solidification a plural number of times poses considerable problems with regard to the yield rate and throughput.
The values of the segregation coefficients mentioned above (6.4×10−6 for iron, 2.8×10−3 for aluminum and 7.37×10−6 for titanium) are obtained with a very slow solidification rate near an approximately equilibrium state, which are called equilibrium segregation coefficients. The segregation coefficient in the actual solidification segregation processing becomes larger than the equilibrium segregation coefficient. As the solidification speed increases, the segregation coefficient becomes larger than the equilibrium segregation coefficient. The segregation coefficient in this case is called an effective segregation coefficient. The effective segregation coefficient “ke” and the equilibrium segregation coefficient “ko” have the relation indicated by the following expression (1):ke=ko/{ko+(1−ko)e−[Rδ/D]}  (1).
From the above expression (1), it is understood that the effective segregation coefficient is determined by the solidification speed R, the thickness δ of the impurity-concentrated layer, and the impurity diffusion coefficient D. The impurity-concentrated layer refers to a portion near the solidification interface where the impurities are concentrated as they are discharged into the molten metal or semiconductor during solidification. The thickness δ of the impurity-concentrated layer is not the actual thickness of the impurity-concentrated layer, but it refers to an imaginary thickness to be used in the expression. From the industrial point of view, it is desired to increase the solidification speed and reduce the effective segregation coefficient, for which it is effective to reduce the thickness of the impurity-concentrated layer.
As a purifying method utilizing solidification segregation, a method of immersing a rotary cooling body into molten silicon and causing high-purity silicon to be crystallized on the outer peripheral surface of the rotary cooling body is disclosed in Japanese Patent Laying-Open No. 63-45112. The method is characterized in that the impurity-concentrated layer is dispersed by stirring the molten metal or semiconductor with rotation of the cooling body, which can increase the solidification speed while maintaining a small segregation coefficient.
Japanese Patent Laying-Open No. 63-45112, however, merely discloses the method of conducting the purifying processing only one time. As described above, it is necessary to carry out purification utilizing solidification segregation two or three times in order to reduce the heavy metal impurity concentrations in metallurgical grade silicon to the level of 0.1 ppmw or less required in the material for solar cells. As such, in order to produce a material for use in solar cells using the relevant method, a way of continuously performing the purifying processing needs to be newly invented.
Meanwhile, considering use for solar cells, elements for determining the conductivity type of silicon among the impurities contained therein need to be most severely controlled in their concentrations, which are typically phosphorus and boron. These elements however have considerably large segregation coefficients on the order of 0.35 and 0.8, respectively, so that it is considered that the purifying method utilizing solidification segregation is hardly effective therefor.
For example, the concentration of phosphorus in metallurgical grade silicon is typically from 30 to 50 ppmw. In order to reduce this concentration to the level of 0.1 ppmw or less required in the material for solar cells, solidification segregation processing will have to be carried out a large number of times. As such, it has been considered that it is very difficult to use the solidification segregation processing for the purpose of eliminating phosphorus, from the industrial point of view.
Thus, as a method of removing phosphorus utilizing a principle other than solidification segregation, a method of melting metallurgical grade silicon under a reduced pressure of 10 Pa or less is disclosed in Japanese Patent Laying-Open No. 6-227808, and a method of irradiating a surface of molten metallurgical grade silicon with an electron beam under a reduced pressure is disclosed in Japanese Patent Laying-Open No. 7-315827. In these methods, the vapor pressure of phosphorus is relatively large, and the evaporation rate of phosphorus is increased by evacuation. In order to process a large amount of molten silicon of high temperature in a vacuum, however, the vacuum pumping facility needs to be increased in size, and the members that can be used within the furnace are restricted, hindering practical use of those methods.
In the 2002 Autumn Meeting of the Japan Institute of Metals (Nov. 3, 2002), it was reported that when the calcium concentration in silicon was set to 0 atomic %, 5 atomic % (7 mass %) and 10 atomic % (14 mass %), the equilibrium distribution coefficient of phosphorus became 0.35, 0.17 and 0.08, respectively.
According to trial calculation based on this report, in the case that the solidification segregation processing is conducted three times with respect to the metallurgical grade silicon having a phosphorus concentration of 30 ppmw, the phosphorus concentration will become 1.3 ppmw, 0.15 ppmw and 0.015 ppmw with the calcium concentrations of 0 mass %, 7 mass % and 14 mass %, respectively. That is, there has been found a possibility that the phosphorus concentration of the level of 0.1 ppmw or less required in the material for solar cells can be achieved by conducting solidification segregation processing two or three times as in the case of removing the heavy metal impurity elements of iron, aluminum and the like, under the condition that calcium is added to silicon at a concentration of 14 mass % or more.
As a method of purifying silicon with calcium added to molten silicon, a method of carrying out acid leaching processing twice after solidification of metallurgical grade silicon added with calcium of 1-10 mass % or 0.3-0.95 mass % is disclosed in U.S. Pat. No. 4,539,194 or in Japanese National Patent Publication No. 2003-516295.
U.S. Pat. No. 4,539,194 is silent about the effect of removing phosphorus. According to Japanese National Patent Publication No. 2003-516295, on the other hand, the change of phosphorus concentration is at most form 52 ppmw to 16 ppmw (effective distribution coefficient: 0.31), which means that the effect of removing phosphorus by this method is small. This is presumably for the following reasons. When a solidified ingot is crushed in order to efficiently carry out acid leaching, the area of grain boundaries appearing on surfaces of the crushed ingot is insufficient, and thus phosphorous that has segregated at the grain boundaries cannot be removed sufficiently. This shows difficulty in removing phosphorus by the acid leaching method.
Patent Document 1: Japanese Patent Laying-Open No. 63-45112
Patent Document 2: Japanese Patent Laying-Open No. 6-227808
Patent Document 3: Japanese Patent Laying-Open No. 7-315827
Patent Document 4: U.S. Pat. No. 4,539,194
Patent Document 5: Japanese National Patent Publication No. 2003-516295